Slagathor
Mechanical
- Jan 6, 2002
- 129
I apologize in advance for the length of this...so please bear with me.
I am an area manager for a mfg of engineered pumping equipment. (I originally started in this industry as a Mech Engineer about 15 years ago). I have a problem involving natural gas engines being started with high inertial loads. This is causing some real problems on a project in progress and I need some guidance.
We are involved in a retrofit of an exiting large pump station for a major water district. The original pumps were supplied by our company about 10 years ago. In their original configuration, the pumps were driven by med/large natural gas engines (about 700 HP). These engines are mounted on cement piers. There are horizontal drive shafts about 8 feet long extending to right angle gear drives. These right angle drives each sit on top of a vertical turbine pump, which extends vertically down into the wet pit below.
These are centrifugal multistage pumps of medium specific speed. They have very low rotational moments of inertia. As such, the reactionary torque load from rotational acceleration is quite low. Centrifugal pump characteristics are such that the steady state torque (from work done by the pump) as a function of speed is a nearly perfectly parabolic. Essentially this means that the steady state torque the pump draws is MUCH lower than the torque rating of the engine right up to you get to full speed.
Because of these characteristics (low inertia and parabolic torque vs Speed), the owner was able to adopt the incorrect practice of starting the engine with the pump coupled to it. They never utilized the clutch. This allowed them to operate the station remotely.
Here is the problem. The pumps are being retrofitted to as new condition, new engines are being installed, and a new COMBO TYPE gear drives are being utilized along with electric motors. The electric motors are an addition to the drive system.
The new configuration is as follows: The new nat gas engines are still mounted on cement piers. There are still horizontal drive shafts about 8 feet long extending to right angle gear drives. These right angle drives sit on top of the vertical turbine pumps, which extend vertically down into the wet pit below. The difference is there are also now large electric motors sitting on top of the combo gear drives.
When the pump is driven by the electric motor, the clutch in the engine is disengaged so you don’t try to turn the engine. When the pump is driven by the motor, however, the induction motor remains engaged to handle the thrust loads of the pump. This means the system now has a LOT of added rotational inertia due to the induction motor.
The owner wants to start them coupled. The new engine mfg (NOT the same mfg as the old engines) is throwing a fit saying you CAN NOT START the engine coupled to the load. This is of course correct. Because of the inertial load, the engine will be in a range of unstable operation, and may not even be able to accelerate. It may even damage the engine.
The problem is the owner did it for 10 years with no issues. Of course they did not have a combo drive with a motor with a huge rotational inertia to spin up. If we blame the increased inertia of the motor, then the owner will likely throw up their hands…and cancel the motors, which gets us into all sorts of contractual issues because the motors are on order and we will have 100% cancellation charges on a bunch of 700 HP induction motors. Not pretty.
The original engine vendor won’t step up and tell the Owner that they were wrong for starting the units coupled all these years because they are sour about losing this bid. The Owner thinks the new engine mfg just wants to sell $300K in fancy clutches.
I need to write a technical memo explaining to the Owner, convincingly, why starting a Natural Gas Engine of this type (or perhaps and internal combustion engine) with a significant inertial load coupled to it is poor practice.
I was looking at getting Internal Combustion Engine Handbook: Basics, Components, Systems, and Perspectives by Richard Van Basshuysen and Fred Schafer for some guidance on the issue. Does anyone have any other recommendations? I am having a hard time finding any technical reference on IC engines with respect to starting loads and unstable operation, etc. I must make the Owner understand that they were never right to start these pump systems coupled. Any guidance is appreciated.
I am an area manager for a mfg of engineered pumping equipment. (I originally started in this industry as a Mech Engineer about 15 years ago). I have a problem involving natural gas engines being started with high inertial loads. This is causing some real problems on a project in progress and I need some guidance.
We are involved in a retrofit of an exiting large pump station for a major water district. The original pumps were supplied by our company about 10 years ago. In their original configuration, the pumps were driven by med/large natural gas engines (about 700 HP). These engines are mounted on cement piers. There are horizontal drive shafts about 8 feet long extending to right angle gear drives. These right angle drives each sit on top of a vertical turbine pump, which extends vertically down into the wet pit below.
These are centrifugal multistage pumps of medium specific speed. They have very low rotational moments of inertia. As such, the reactionary torque load from rotational acceleration is quite low. Centrifugal pump characteristics are such that the steady state torque (from work done by the pump) as a function of speed is a nearly perfectly parabolic. Essentially this means that the steady state torque the pump draws is MUCH lower than the torque rating of the engine right up to you get to full speed.
Because of these characteristics (low inertia and parabolic torque vs Speed), the owner was able to adopt the incorrect practice of starting the engine with the pump coupled to it. They never utilized the clutch. This allowed them to operate the station remotely.
Here is the problem. The pumps are being retrofitted to as new condition, new engines are being installed, and a new COMBO TYPE gear drives are being utilized along with electric motors. The electric motors are an addition to the drive system.
The new configuration is as follows: The new nat gas engines are still mounted on cement piers. There are still horizontal drive shafts about 8 feet long extending to right angle gear drives. These right angle drives sit on top of the vertical turbine pumps, which extend vertically down into the wet pit below. The difference is there are also now large electric motors sitting on top of the combo gear drives.
When the pump is driven by the electric motor, the clutch in the engine is disengaged so you don’t try to turn the engine. When the pump is driven by the motor, however, the induction motor remains engaged to handle the thrust loads of the pump. This means the system now has a LOT of added rotational inertia due to the induction motor.
The owner wants to start them coupled. The new engine mfg (NOT the same mfg as the old engines) is throwing a fit saying you CAN NOT START the engine coupled to the load. This is of course correct. Because of the inertial load, the engine will be in a range of unstable operation, and may not even be able to accelerate. It may even damage the engine.
The problem is the owner did it for 10 years with no issues. Of course they did not have a combo drive with a motor with a huge rotational inertia to spin up. If we blame the increased inertia of the motor, then the owner will likely throw up their hands…and cancel the motors, which gets us into all sorts of contractual issues because the motors are on order and we will have 100% cancellation charges on a bunch of 700 HP induction motors. Not pretty.
The original engine vendor won’t step up and tell the Owner that they were wrong for starting the units coupled all these years because they are sour about losing this bid. The Owner thinks the new engine mfg just wants to sell $300K in fancy clutches.
I need to write a technical memo explaining to the Owner, convincingly, why starting a Natural Gas Engine of this type (or perhaps and internal combustion engine) with a significant inertial load coupled to it is poor practice.
I was looking at getting Internal Combustion Engine Handbook: Basics, Components, Systems, and Perspectives by Richard Van Basshuysen and Fred Schafer for some guidance on the issue. Does anyone have any other recommendations? I am having a hard time finding any technical reference on IC engines with respect to starting loads and unstable operation, etc. I must make the Owner understand that they were never right to start these pump systems coupled. Any guidance is appreciated.
